This invention relates to a digital transmission system which converts information to the form of a pulse train, multiplexes an overhead signal with the pulse train, transmits the pulse train, reproduces the clock on the receiver side and demultiplexes or re-multiplexes the received signals.
Digital transmission systems have generally the structure such as shown in FIG. 14. Input signals 51, are fed to a multiplexer, MUX, 52, where a frame signal and the like are generated and all of them are multiplexed into one pulse stream or train and are then fed to an office repeater terminal 55 through a transmission line 53, where signals 54, e.g., an alarm signal, an order wire signal, etc, necessary for transmission are further added. The signals are then delivered to a transmission line 56. If the transmission distance is long, the attenuated signal is amplified by an intermediate repeater 57 and then delivered to a next transmission line 58. After extracting the alarm signal, the order wire signal, etc, 54 from the signal pulse train, an office repeater 59 on the receiver side transmits the signal pulse train to a demultiplexer, DEMUX 62, through a transmission line 61. The signals 51 thus separated are transferred to other apparatuses, such as switching equipment 65 through a transmission line 63 for further use, for example on transmission line 64.
The fundamental constituent elements of such a digital transmission system can be divided into a transmission processing circuit and a multiplexing-demultiplexing conversion circuit, as shown in FIG. 15. In other words, the received pulse train 81 is sent as a reshaped signal 83 by a reshaping circuit 82, to a clock recovery circuit, CLK CKT 84, and to a decision and regeneration circuit, CKT 86. In the clock recovery circuit the clock pulse 85 is recovered from the reshaped input signal pulse train 83 and is used for decision and regeneration of the signal in the decision and regeneration circuit 86 and for multiplexing-demultiplexing and conversion in the multiplexing-demultiplexing circuit, MUX-DEMUX 88. The transmission processing is conducted by the reshaping circuit 82, the clock recovery circuit 84 and the decision and regeneration circuit 86, and the signal pulse train 81 that is attenuated and inputted is outputted as a regenerated pulse train 87.
In the multiplexing-demultiplexing conversion circuit 88, on the other hand, generation of a frame for multiplexing the signals, detection of a frame pulse for demultiplexing the multiplexed signals, multiplexing-demultiplexing of an alarm signal, an order wire signal, etc, necessary for maintenance and operation of signal transmission, line coding necessary for transmission processing, detection of code errors, and the like, are carried out. Input and output signals 89, 90 are housekeeping signals such as the alarm and order wire signals, for example, and main information signals 91 may be voice or video signals.
The transmission processing and multiplexing-demultiplexing conversion are necessary irrespective of the transmission distance. In other words, these processings are necessary not only after each of the transmission lines 56 and 58 which generally have a relatively large distance but also after each of the transmission lines 53, 61, 63, 64 having a relatively short distance inside the office. The reason why such processings are necessary in the short distance zone is mainly because clock recovery is necessary. So-called "external timing" which sends the clock signals by a separate system is not preferred because a manual operation such as phase adjustment becomes necessary.
Sometimes, clock recovery becomes necessary in the transmission zone having a smaller distance than the intra-office transmission, such as transmission inside the apparatus. When the signal processing speed becomes extremely high as in the recent systems, crosstalk between the wirings inside the apparatus cannot be neglected and could cause problems in many cases. In such a case, it is necessary either to extract the clock and use it to regenerate the signal, or to make use of an optical fiber for the wirings, because optical fibers do not suffer from cross-talk.
In the preceding discussion, electrical processing was considered. Investigation has been started recently in optical fiber communication to process light directly (all optical transmission) so as to realize economical ultra-high speed transmission without converting light into electric signals as has been made in the prior art technique.
In such a case, in the optical domain, (1) simplification of the processing of the multiplexing frame and (2) simplification of the clock recovery become common problems for accomplishing the system.
A typical example of the signal processing circuit for processing light in the optical domain without converting optical information to an electric signal is an optical logic device using a multi quantum well etalon of GaAs/AlGaAs such as shown in FIG. 3a. The device comprises a multiple quantum well 45 consisting of a GaAs substrate 41, reflecting mirrors 42 and alternate layers of GaAs 43 and AlGaAs 44. The quantum well 45 has non-linear characteristics such that its output Z is turned ON and OFF depending on whether the sum of input light X and Y exceeds a predetermined value (see FIG. 3b). Functions such as logical sum (OR), logical product (AND), bi-stable operation, and the like, for optical signals can be accomplished by use of such a device, but it has been difficult to provide it with diversified functions such as exclusive-OR operation in the same way as in the conventional electrical circuit.